Datasheet

LM3424
www.ti.com SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
INPUT CAPACITORS
The input capacitance (C
IN
) provides energy during the discontinuous portions of the switching period. For buck
and buck-boost regulators, C
IN
provides energy during t
ON
and during t
OFF
, the input voltage source charges up
C
IN
with the average input current (I
IN
). For boost regulators, C
IN
only needs to provide the ripple current due to
the direct connection to the inductor. C
IN
is selected given the maximum input voltage ripple (Δv
IN-PP
) which can
be tolerated. Δv
IN-PP
is suggested to be less than 10% of the input voltage (V
IN
).
An input capacitance at least 100% greater than the calculated C
IN
value is recommended to account for derating
due to temperature and operating voltage. When PWM dimming, even more capacitance can be helpful to
minimize the large current draw from the input voltage source during the rising transistion of the LED current
waveform.
The chosen input capacitors must also have the necessary RMS current rating. Ceramic capacitors are again the
best choice due to their high ripple current rating, long lifetime, and good temperature performance. An X7R
dieletric rating is suggested.
For most applications, it is recommended to bypass the V
IN
pin will an 0.1 µF ceramic capacitor placed as close
as possible to the pin. In situations where the bulk input capacitance may be far from the LM3424 device, a 10 Ω
series resistor can be placed between the bulk input capacitance and the bypass capacitor, creating a 150 kHz
filter to eliminate undesired high frequency noise.
MAIN MosFET / DIMMING MosFET
The LM3424 requires an external NFET (Q1) as the main power MosFET for the switching regulator. Q1 is
recommended to have a voltage rating at least 15% higher than the maximum transistor voltage to ensure safe
operation during the ringing of the switch node. In practice, all switching regulators have some ringing at the
switch node due to the diode parasitic capacitance and the lead inductance. The current rating is recommended
to be at least 10% higher than the average transistor current. The power rating is then verified by calculating the
power loss given the RMS transistor current and the NFET on-resistance (R
DS-ON
).
When PWM dimming, the LM3424 requires another MosFET (Q2) placed in series (or parallel for a buck
regulator) with the LED load. This MosFET should have a voltage rating equal to the output voltage (V
O
) and a
current rating at least 10% higher than the nominal LED current (I
LED
) . The power rating is simply V
O
multiplied
by I
LED
, assuming 100% dimming duty cycle (continuous operation) will occur.
In general, the NFETs should be chosen to minimize total gate charge (Q
g
) when f
SW
is high and minimize R
DS-ON
otherwise. This will minimize the dominant power losses in the system. Frequently, higher current NFETs in
larger packages are chosen for better thermal performance.
RE-CIRCULATING DIODE
A re-circulating diode (D1) is required to carry the inductor current during t
OFF
. The most efficient choice for D1 is
a Schottky diode due to low forward voltage drop and near-zero reverse recovery time. Similar to Q1, D1 is
recommended to have a voltage rating at least 15% higher than the maximum transistor voltage to ensure safe
operation during the ringing of the switch node and a current rating at least 10% higher than the average diode
current. The power rating is verified by calculating the power loss through the diode. This is accomplished by
checking the typical diode forward voltage from the I-V curve on the product datasheet and multiplying by the
average diode current. In general, higher current diodes have a lower forward voltage and come in better
performing packages minimizing both power losses and temperature rise.
CIRCUIT LAYOUT
The performance of any switching regulator depends as much upon the layout of the PCB as the component
selection. Following a few simple guidelines will maximimize noise rejection and minimize the generation of EMI
within the circuit.
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